The 24-year-old Japanese became the first Asian man to reach a Grand Slam final with a stunning 6-4, 1-6, 7-6 (7/4), 6-3 semi-final victory over world number one Novak Djokovic.
But having been forced to skip the crucial warm-up Masters events in Toronto and Cincinnati, the Japanese star said he wasn't sure if he would be able to play at Flushing Meadows.
"I just started playing points a few days ago before the tournament. I didn't even know if I should come to New York, so I wasn't expecting anything actually," said the 10th seeded Nishikori after recording his second career win over seven-time major winner and reigning Wimbledon champion Djokovic.
"But after playing the first and second match here I got more confidence in my foot and there was no pain.
"I was sliding a little more. But my tennis was there already. I tried to play one match at a time and now I'm here. Maybe I should rest three weeks before a Grand Slam."
Nishikori has endured a gruelling route to his first major final, playing two five-set matches and spending eight and a half hours on court in beating Milos Raonic and Australian Open champion Stan Wawrinka in the fourth round and quarter-finals respectively.
He admitted that in the past he may have struggled to get through such examinations, and his stamina and fitness were often questioned after he fell victim to elbow, back and groin injuries at various stages of his career.
"A few years ago it would have been tough. But having the last two days off has helped as it wasn't easy playing two five sets and four-hour matches," said Nishikori, who, like Djokovic, struggled in on-court temperatures which were closing in on 40 degrees.
"It was even tough for me to play today, especially the third and fourth sets I couldn't really put effort for every game.
"In the last game, I tried to get more energy and tried to concentrate again. I have been doing that well compared to a couple of years ago. I hope I recover and be ready for Monday and the final."
- 'Anything can happen' -
Nishikori also praised the impact of coach Michael Chang, the 1989 French Open champion, who has been working in tandem with long-term handler Dante Bottini.
"It's been really helpful. He's helped me a lot from the end of last year. I feel my tennis is more aggressive and I am playing with more confidence. He's tough but I sometimes need that, I need people who can push me."
Chang said he counseled Nishikori "get through the first two matches -- then anything can happen."
He told reporters: "I gave him examples of situations before that I've known, for myself and for Pete Sampras, where they weren't quite sure they were going to play and then ended up doing great.
"I actually ended up getting to finals of the French in '95 after coming very, very close to pulling out before the tournament."
Chang added that Nishikori sitting out Toronto and Cincinnati was probably a blessing.
"The good thing is that he's had so much time off. He's fresh, he's hungry, he's eager," said Chang.